Th. Krist

The polarizing beam splitter guide at BENSC

Abstract A polarizing beam-splitter guide, theoretically described earlier, was installed in the neutron guide hall at BENSC in the winter of 1993/94. It splits the incoming neutron beam with wavelengths beyond 2.5 A into two oppositely polarized beams. The active elements of the splitter system are Si CoFe supermirrors deposited on Si wafers, which show a good polarization capability between the critical angle of Si of 0.047° per A neutron wavelength and a cut-off angle of 0.21° per A, i.e. twice the critical angle of natural Ni. For single mirrors in the supermirror region an average reflection coefficient of 90% was achieved at a polarization of 96% for the reflected beam while the transmission coefficient amounted to 80% for a polarization of 97%. These values apply for a magnetizing field of 300 G and improve slightly for higher fields. Preliminary results for the whole guide show a polarization above 92% for neutrons with a wavelength of 3 A.

NEW COMPACT NEUTRON POLARIZER

Abstract A new type of a neutron polarizing bender was developed in co-operation with BENSC and ANSTO. It is based upon bent thin silicon wafers coated on one side with SiFeCo polarizing supermirrors and on the other side with Gd. Initial tests at BENSC in a 300 Oe magnetic field yielded a transmission of spin-up neutrons of about 55% over an angle range of 0.75° and flipping ratios > 30. Subsequent tests at ANSTO at 1200 Oe yielded a transmission of 48% with a flipping ratio > 45.

Polarizing Ti1-uXu/FexCoyVz supermirrors

Abstract Polarizing multilayers and supermirrors composed of Ti1−uXu and FexCoyVz have been produced using DC-magnetron sputtering. The structure of the samples has been characterized by neutron and X-ray diffraction. The results show that the smoothness of the layers can be improved by adding controlled amounts of alloying materials, an effect that has already been observed for Ni/Ti supermirrors. Flipping ratios of more than 40 and reflectivities of more than 85% at twice the critical angle of Ni are obtained, depending on the concentrations of the materials, u, x, y, z. The sputtering process can be optimized in such a way that the magnetization within the plane is anisotropic, thus allowing a saturation of the samples in low fields.

Non-specular reflectivity of spin flipped neutrons

Abstract For the first time we observed large intensity non-specular reflection peaks of neutrons incident at small grazing angles on not fully magnetized ferromagnetic samples. In addition to the normal specular reflection spin flipped neutrons are reflected at distinctly higher (lower) angles if the incoming spin down (spin up) neutrons hit the surface at angles near the respective critical angle. The same effect occurs in transmission. The angles at which the non-specular reflection occurs can be calculated by a simple model, which assumes a spin flip without a change of direction and takes into account the different refraction angles of the original and the flipped spin state when the neutrons leave the sample.

High-efficiency transmission neutron polarizer for high-resolution double crystal diffractometer

Abstract An efficient transmission geometry neutron polarizer for the high-resolution double crystal diffractometer at HMI ( λ = 4.8 A ) is described. A polarization of about 94% was achieved and the polarized neutron beam intensity amounts to 40% of the nonpolarized beam intensity. This opens up wide possibilities for the study of magnetic small-angle scattering for extremely small momentum transfer ( Q ∼ 10 −5 A −1 ).

Inversely polarizing CoCu neutron supermirrors

Abstract A new type of a neutron polarizing supermirror was developed which reflects spin-down neutrons, contrary to all hitherto produced polarizing supermirrors which reflect spin-up neutrons. Reflectivities of 80–90% and polarizations up to 96% were achieved in a scattering vector interval from 0.176 to 0.35 nm−1.

Monte Carlo simulation of polarising cavities

Abstract High-reflectivity supermirrors allow for sophisticated neutron optical systems working with multiple reflections (for example polarising cavities and beam splitters). Polarising cavities are highly efficient broad-wavelength band-polariser systems. Beam-splitter polarising cavities, for example, yield a practically ideal separation into one spin “up” and one spin “down” neutron beam, which can supply two different instruments. This later principle was already implemented at BENSC. Monte Carlo simulations were recently done to optimise polarising cavity systems and to search for possible new applications at ESS. Wavelength-dependent polarisation intensities were computed with ESS moderators.

Interface effects in FeCoSi magnetic multilayers

Abstract The performance of neutron supermirrors is strongly influenced by interface effects, especially if magnetic layers are involved. We determined the thickness and composition of the interface layer evolving during sputtering Fe89Co11Si polarizing supermirrors. A set of multilayers with varying thicknesses of the FeCo layer was produced and characterized by SQUID magnetometry, Rutherford backscattering, X-ray diffraction and polarized neutron reflection. It was found that at each interface two atomic layers of the FeCo alloy formed a silicide, thereby loosing magnetization.